从源码看java中Integer的缓存问题

1 public static void compare1(){
2         Integer i1 = 127, i2 = 127, i3 = 128, i4 = 128;
3         System.out.println(i1  == i2);
4         System.out.println(i1.equals(i2));
5         System.out.println(i3  == i4);
6         System.out.println(i3.equals(i4));
7     }

 1 /**
 2      * Returns an {@code Integer} instance representing the specified
 3      * {@code int} value.  If a new {@code Integer} instance is not
 4      * required, this method should generally be used in preference to
 5      * the constructor {@link #Integer(int)}, as this method is likely
 6      * to yield significantly better space and time performance by
 7      * caching frequently requested values.
 8      *
 9      * This method will always cache values in the range -128 to 127,
10      * inclusive, and may cache other values outside of this range.
11      *
12      * @param  i an {@code int} value.
13      * @return an {@code Integer} instance representing {@code i}.
14      * @since  1.5
15      */
16     public static Integer valueOf(int i) {
17         if (i >= IntegerCache.low && i <= IntegerCache.high)
18             return IntegerCache.cache[i + (-IntegerCache.low)];
19         return new Integer(i);
20     }

 1 /**
 2      * Cache to support the object identity semantics of autoboxing for values between
 3      * -128 and 127 (inclusive) as required by JLS.
 4      *
 5      * The cache is initialized on first usage.  The size of the cache
 6      * may be controlled by the {@code -XX:AutoBoxCacheMax=<size>} option.
 7      * During VM initialization, java.lang.Integer.IntegerCache.high property
 8      * may be set and saved in the private system properties in the
 9      * sun.misc.VM class.
10      */
11 
12     private static class IntegerCache {
13         static final int low = -128;
14         static final int high;
15         static final Integer cache[];
16 
17         static {
18             // high value may be configured by property
19             int h = 127;
20             String integerCacheHighPropValue =
21                 sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
22             if (integerCacheHighPropValue != null) {
23                 try {
24                     int i = parseInt(integerCacheHighPropValue);
25                     i = Math.max(i, 127);
26                     // Maximum array size is Integer.MAX_VALUE
27                     h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
28                 } catch( NumberFormatException nfe) {
29                     // If the property cannot be parsed into an int, ignore it.
30                 }
31             }
32             high = h;
33 
34             cache = new Integer[(high - low) + 1];
35             int j = low;
36             for(int k = 0; k < cache.length; k++)
37                 cache[k] = new Integer(j++);
38 
39             // range [-128, 127] must be interned (JLS7 5.1.7)
40             assert IntegerCache.high >= 127;
41         }
42 
43         private IntegerCache() {}
44     }

 1 /**
 2      * Compares this object to the specified object.  The result is
 3      * {@code true} if and only if the argument is not
 4      * {@code null} and is an {@code Integer} object that
 5      * contains the same {@code int} value as this object.
 6      *
 7      * @param   obj   the object to compare with.
 8      * @return  {@code true} if the objects are the same;
 9      *          {@code false} otherwise.
10      */
11     public boolean equals(Object obj) {
12         if (obj instanceof Integer) {
13             return value == ((Integer)obj).intValue();
14         }
15         return false;
16     }